1. Field of the Invention
The invention generally relates to gamma ray detectors. More particularly, the invention relates to a gamma ray detector for determining the positions of gamma ray interactions for producing an image of a scanned object.
2. Background
Gamma ray detectors are used in a wide variety of apparatus, such as in positron emission tomography (PET), single photon emission-computed tomography (SPECT), contraband explosive detectors, and the like. All of such apparatus depend upon detectors that can determine the position of interactions of gamma rays with the detectors, such that with a plurality of such position measurements, a scan of an object of interest can be made. These techniques are well known in the art and need not be detailed herein.
A difficulty encountered with these detectors is that in order to perform a scan, a multiplicity of such detectors are necessary, and the positions of interactions of gamma rays with the detectors must be determined so that with a plurality of such determinations (e.g., in the millions) sufficient data is obtained to produce an accurate scan image of the object of interest. Since each detector must be capable of generating position data for a gamma ray interaction, acquisition of such position data and the compilation thereof (e.g., by a computer) requires very substantial and expensive apparatus. Typically, the data of such detectors is initiated by an interaction of a gamma ray with a scintillator material of the detector that generates a light. By determining the detector in which such light was emitted, and the position of that emitted light within the detector, a data point for a scan is produced. By providing a multiplicity of such detectors, a multiplicity of data points can be acquired. A computer can then resolve the data into an image of the object of interest being scanned.
The usual detector for such gamma ray scanning devices is an inorganic scintillating crystalline material (e.g., cerium doped lutetium oxyorthosilicate (LSO) and bismuth germinate (BGO)) which is, in and of itself, expensive. The crystalline material is a scintillator material, which will emit light, and therefore the position of interaction of a gamma ray can be determined. The X-Y position resolution of such detectors is typically 20 square millimeters and typically is not uniform for all positions, and this leaves a basic Inaccuracy in not knowing precisely where in the detector (i.e., in the X and Y coordinates) that interaction occurred. In addition, the depth of the interaction (i.e., the Z coordinate) is generally not determined, or is poorly determined, resulting in a so-called parallax error and further image inaccuracy. Those effects result in less than desirable accuracy of scan images for the object of interest.
A modular light signal triggerable detector is disclosed in Bryman, Douglas, U.S. Pat. No. 6,100,532, Detector for Gamma Rays (Aug. 8, 2000) which is hereby incorporated by reference in its entirety. This patent discloses a gamma ray detector for determining the position of gamma ray interactions. The detector has at least one module, and each module has a converter for converting gamma rays into charged particles. A scintillator is provided for emitting light in response to the charged particles produced by the converter. A photodetector determines when light has been emitted from the scintillator. A two-coordinate position detector is provided for determining the X, Y and Z coordinates of charged particles interacting with the position detector. A controller and signal device is provided for signaling the presence of emitted light in the photodetectors and for activating the position detector. This system addresses some of the above-noted deficiencies and provides a gamma ray detector which can be inexpensively constructed, requires far less monitoring instrumentation for acquisition of the required data, and which can determine the X, Y and Z coordinates of the gamma ray interaction.
The conversion of gamma rays in material (including heavy liquids like xenon (Xe), krypton (Kr), and the like) and the production of scintillation light and charged products (electrons and positrons) are well studied and understood by those skilled in the art. Further, software tools are available that simulate the interactions of gamma rays and charged particles with the matter. Position sensitive detectors for charged particles, such as noble liquid ionization chambers, time-projection-chambers (TPC), and light detection arrays are commonly used instruments which are known to have position and energy resolution capability similar to those obtained in the present application.
Liquid Xe position sensitive ionization detectors with grids such as described by K. Masuda et al., A Liquid Xenon Position Sensitive Gamma-Ray Detector for Positron Annihilation Experiments, Nucl. Instr. Meth. 188 (1981) 629-638; and K. Masuda, et al., Test of a Dual-Type Gridded Ionization Chamber Using Liquid Zenon, Nucl. Instr. Meth. 174 (1980) 439-446, each of which is hereby incorporated by reference in its entirety, are known to be able to provide sub-millimeter position resolution for low energy gamma rays. A gated time projection ionization chamber has been reported in the articles describing the TRIUMF TPC (gas drift device). The Columbia University liquid Xe TPC (E. Aprile, et al., The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) for Medium Energy Astrophysics, Proceedings-SPIE The International Society For Optical Engineering, SPIE Vol. 2806, pgs. 337-348, which is hereby incorporated by reference in its entirety, is an example of a liquid Xe ionization TPC that achieved 1 mm position resolution and energy resolution of 5.9% at 1 MeV gamma ray energy. Additionally, Lopes et al. have constructed a liquid Xe ionization detector with transaxial position resolution of 1mm, depth of interaction resolution of 5 mm, coincidence time resolution of 1.3 ns, energy resolution at 511 keV of 17% and efficiency of 60% (see, M. Lopes, et al., Positron Emisson Tomography Instrumentation: Development of a Detector Based on Liquid Xenon, Proc. Calorimetry in High Energy Physics, pages 675-680 (1999)), which is hereby incorporated by reference in its entirety.
These and numerous other articles present example solutions for instrumentation of the ionization signal collection using pads and wires, gating grids and scintillator triggers that are applied to the problem of measuring charged particle trajectories. In these instruments the scintillation light has been used primarily as a fast indicator that a suitable event has occurred without specifically localizing the point of interaction.
In the KAMIOKANDE (as described in K.S. Hirata et al., Experimental Study of the Atmospheric Neutrino Flux, PHYSICS LETTERS B, Vol. 205, number 2,3, p. 416-420 (1988)) and other detectors, arrays of photodetectors covering the surface of light-emitting liquids and solids have been used to localize the position of interactions of gamma rays and charged particles. In L. Barkov et al., Search for μ+→e+γ down to 10−14 branching ratio, Paul Scherer Institute proposal R-99-05.1 (1999), which is hereby incorporated by reference in its entirety) to study lepton-flavor-violating decay μ+→e+γ, a liquid Xe scintillation detector using an array of photo-multiplier tubes surrounding a small volume has been demonstrated to give 0.8 cm full width half maximum (fwhm) position resolution for 1 MeV gamma rays.